topic 2

• Electrical Charge


  For charge to flow, circuit must be closed (no open switches) and there must be a source of potential difference (battery/cell)
• Electrical Current


  • Current is the flow of electrical charge
  • Greater the rate of flow of charge, greater current
• In a single closed loop, the current has the same value at any point
• Resistors


  • If the resistance is constant, an ohmic conductor, current is directly proportional to the potential difference, in this case the graph is linear
  • If the resistance of components such as lamps, diodes, thermistors and LDRs is not constant it changes with the current through the component, so the graph is nonlinear
• How resistance changes with current

  1. As current increases, electrons (charge) has more energy

  2. When electrons flow through a resistor, they collide with the atoms in the resistor

  3. This transfers energy to the atoms, causing them to vibrate more

  4. This makes it more difficult for electrons to flow through the resistor

  5. So resistance increases, and current decreases
• How resistance changes with temperature

  1. Normal wires - atoms vibrate when hot, making it more difficult for electrons to flow

  2. Thermistor - in hotter temperatures the resistance is lower
• Resistance and length


  The greater the length, the more resistance and the lower the current
• Resistance and light


  • The greater the intensity of light, the lower the resistance
  • The resistance is greatest when it is dark
• Resistance and voltage


  Diodes allow current to flow freely in one direction, in the opposite direction they have a very high resistance so no current can flow
• Series Circuits


  • Closed circuit
  • Current only follows a single path
  • The current is the same everywhere
  • Total Resistance = R1 + R2 + ...
• Parallel Circuits


  • Branched circuit
  • Current splits into multiple paths
  • Total current into a junction = total current in each of the branches
  • The potential difference is the same across each "branch"
  • The total resistance for two resistors in parallel is less than the resistance of the smallest resistor
• Differences between Series and Parallel Circuits

  • Series: Components are connected end to end, all the current flows through all the components, can only switch them all off at once, potential difference is shared across the whole circuit, current is the same through all parts of the circuit, total resistance is the sum of the resistance in each component
  • Parallel: Components are connected separately to the power supply, current flows through each one separately, you can switch each component off individually, potential difference is the same across all branches, current is shared between each of the branches, total resistance is less than the branch with the smallest resistance
• Mains electricity


  • AC supply, in the UK has a frequency of 50 Hz and is about 230 V
  • AC is alternating current, which comes from the mains, current continuously varies, from positive to negative (charge changes direction)
  • DC, direct current, is the movement of charge in one direction only, from cells and batteries
• Wires in a plug

  • Live wire - brown, at 230V, carries the alternating potential difference from the supply
  • Neutral wire - blue, at 0V, completes the circuit
  • Earth wire - green and yellow stripes, at 0V, safety wire to stop the appliance becoming live
• National Grid


  • System of cables and transformers linking power stations to consumers across the UK
  • Step-up transformers increase the potential difference from the power station to the National Grid, so current decreases and less energy is lost
  • Step-down transformers decrease the potential difference from the National Grid to consumers, for consumer safety
• Charge


  • Property of all matter
  • Positive and negative charges exist, if a body has the same amount of positive and negative charge, they cancel out, forming a neutral body
  • Like charges repel, opposite charges attract
• Insulators


  Their electrons cannot flow throughout the material, they are fixed
• Conductors


  Their electrons can flow, and are not fixed (they are delocalised)
• Static Electricity


  1. When two insulators are rubbed together, electrons are transferred from one object to the other, forming a positive charge on one object and a negative charge on the other

  2. If conductors were rubbed, electrons will flow in/out of them cancelling out any effect, so they stay neutral

  3. Sparking occurs when enough charge builds up, and the objects are close but not touching
• Electrostatic forces


  • The charged objects experience a force – electrostatic force (of attraction/repulsion)
  • Greater charge = greater force
  • Closer together = greater force (force is proportional to the inverse square of the distance)
  • It is a noncontact force, as force can be felt even when the objects are not touching
• Electric Fields


  • They point in the direction a positive charge would go i.e. away from positive charges, and towards negative charges
  • They point to charges at right angles to the surface
  • Stronger the charge, the more field lines present and the stronger the force felt
  • Closer to the charge, the stronger the force felt